Q.3: Explain the process of breathing ?
Answers
Explanation:
- The process of breathing (respiration) is divided into two distinct phases, inspiration (inhalation) and expiration (exhalation). During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward.
Answer:
Control of respiration
Although breathing is simple mechanically, its control is complex. The respiratory control centre is situated in the medulla oblongata of the brain. This sets the rhythm of breathing and contains neurons that are self-excitatory (rather like the cells in the sino-atrial node in the heart) and which fire off in a cycle. This maintains the normal respiratory rate of 12-15 breaths a minute.
When the inspiratory neurons in the medulla fire, they excite the muscles of inspiration - the phrenic nerve to the diaphragm and the intercostal nerves to the intercostal muscles - causing them to shorten and enlarge the volume of the chest cavity. When the medullary neurons stop firing, the muscles recoil and the chest cavity returns to its resting size.
During exercise we need to deliver more oxygen than normal to the tissues. The brain centres send more impulses to the respiratory muscles and we breathe more deeply and quickly. During forced expiration, areas in the medulla fire off impulses that contract the muscles of forced expiration - abdominal muscles and the internal intercostals.
A number of factors influence the rate and depth of respiration:
- We have a limited amount of voluntary control over respiration. For example, we can control expiration while talking or singing;
- If the lungs begin to overinflate, stretch receptors in the bronchioles and alveoli are triggered and switch off the respiratory centre so that air is expelled and the lungs return to normal;
- Chemical factors play a very important role. Blood pH and levels of oxygen and carbon dioxide are constantly monitored by specialised chemoreceptors. A rise in CO2 and a resultant fall in pH will increase the rate and depth of breathing, so that CO2 is blown off and the levels return to normal. These changes seem to act directly on the medullary centres. A fall in blood oxygen levels also sends impulses to the medulla to increase the rate and depth of breathing but usually only when they are very low.